The volatile gas components present in a breath of an individual may reveal a health condition of the individual. For example, patients having diabetes, renal failure, or high cholesterol may have high concentrations of acetone, ammonia, or isoprene respectively in their breath. The reactive oxygen species oxidize polyunsaturated fatty acids, excreting lipid-based free radicals and eventually volatile alkanes and methylated alkanes in breath as markers for oxidative stress. Therefore, breath analysis is a way of screening patients for early detection of certain diseases by a reliable, noninvasive, painless, and inexpensive approach. The types and concentrations of chemical compounds that might serve as biomarkers for particular diseases can be determined by various methods, such as gas chromatography, laser spectrometry, ion mobility spectrometry, or sensor technologies such as electrochemical sensor, photo ionization sensor and semiconductor sensor. The latest advancement in breath analysis is to use photo ionization detector (PID) for screening patients having certain diseases.
A typical PID includes an ionization source with high-energy photons, an ionization chamber, and an ion detector. The PID can detect volatile organic gases. In a PID, the high-energy photons are directed to the ionization chamber for collisions with gas molecules, wherein the photons ionize the gas molecules if the energy of the photon is larger than the ionization potential of the molecule. The ionized molecules are electrically detectable as ions and electrons.
Although PID-based sensors may be used to correlate a response signal with a change in ionization, such response signals may be deleteriously affected by other, interfering signals, thereby generating signal artifacts. The signal artifacts may also include unwanted signal responses, for example, responses generated from one or more interfering molecules, such as water, ethanol, or carbon monoxide. The molecules are referred to herein as interfering substances, have high ionization potentials and can block or absorb UV photons, which decrease the detector sensitivity. A concern in prior PIDs is contaminants introduced with the sample and metal atoms released from internal and external electrodes which can deposit on the optical window of the UV lamp forming a coating and reduce the intensity of the UV light from the lamp. The coating reduces the sensitivity of the PID and requires recalibration of the PID using samples of known concentrations of detectable gas. Conventional PIDs also suffer from unstable baseline currents because of metallic electrodes, which are exposed to high-energy photons and release free electrons, which can produce a baseline current flow even when no ionizable gases are present. Frequent calibration of the PID with a reference is needed in order to re-establish a correct baseline current.
Therefore, it is desirable to have a PID sensor, which can detect chemical components in breath with high selectivity and efficiency. The PID is desired to be energy efficient, can be miniaturized for portable applications, does not require frequent calibrations, and has a quick response to change in concentrations of volatile gases in surroundings.